You could use a set of shared variables - one for each subreport. Declare them and set their initial values to 0 in the main report. Something like this:
BeforeReadingRecords;
Shared NumberVar sub1 := 0;
Shared NumberVar sub2 := 0;
...
Shared NumberVar sub13 := 0;
In each subreport, you would use the corresponding shared variable that's declared in the main report. So in Subreport 1, you would have something like this:
Shared NumberVar sub1;
sub1 := sum({table.field});
Repeat for each variable and subreport.
In your main report, you would then have another formula that would add all of the values of these shared variables together:
Shared NumberVar sub1;
Shared NumberVar sub2;
...
Shared NumberVar sub13;
sub1 + sub2 + ... + sub13
Put this final formula on the report to display your total.
Note that you MUST declare the variables in EVERY formula where they're used. It's the only way that Crystal knows how to use them.
-Dell